This past week Twitter has severally changed the API procedures for other developers to develop software to connect with Twitter. Unfortunatly this means that if you use a 3rd party application like TweetDeck, pretty soon you will no longer be able to. In fact TweetDeck has announced that in May the application will no longer be available for download and shortly after that it was no longer function.
There is plenty of built in features into iOS and the mac platform that will allow integrated Twitter functions within the OS. Also for the iPhone the Twitter App will still remain available to download.